1. Field of the Invention
This invention relates to a printing ink container comprising a cylinder provided with an ink discharge port at its leading end and a piston received in the cylinder to be slidable along the inner surface of the cylinder, ink being filled into the space defined by the cylinder and the piston.
2. Description of the Related Art
There has been known a printing ink container comprising a cylinder provided with an ink discharge port at its leading end and a piston received in the cylinder to be slidable along the inner surface of the cylinder. Ink is filled into the space defined by the cylinder and the piston. An elastic ink scraper portion is mounted along the rim of the piston. As the ink in the container is consumed, the piston slides toward the ink discharge port under the atmospheric pressure. When the piston slides toward the ink discharge port, the ink scraper portion scrapes ink off the inner surface of the cylinder.
In a printer, such an ink container is generally mounted to be removable from the printer body, and when the ink in the ink container is consumed, the ink container is replaced with a new refill (a disposable type) or the ink container is removed from the printer body, refilled with ink and then returned to the printer body (a reusable type).
In the case of a disposable type ink container, empty containers may be recycled to make material for other plastic products.
Which ever type is employed, it is necessary to watch the remainder of ink in the ink container, or the ink can suddenly run out to force the printer to be stopped until the ink container is replaced with a new refill or the ink container is refilled with ink. That the time efficiency is high is a strong point of a stencil printer. However when the ink suddenly runs out to force the printer to be stopped until the ink container is replaced with a new refill or the ink container is refilled with ink, such a strong point of the printer is hurt. Accordingly, it is necessary that the ink is about to be exhausted is recognized at least immediately before the ink actually runs out.
This problem can be overcome in the simplest way by the user visually watching the remainder of ink. However since the ink container is generally placed deep in the printer, the user must check the remainder of ink by taking out the ink container and opening the cap with the printer stopped. If the ink container is of transparent or semitransparent material, the user can check the remainder of ink with the cap kept on. However these actions are troublesome to the user. Accordingly, systems for detecting that the remainder of ink in the ink container becomes small have been proposed or have been put into practice.
For example, there has been proposed a system in which a light emitter is positioned on one side of a semitransparent ink container with a plurality of light receivers positioned on the opposite side of the ink container so that when ink exists between a combination of the light emitter and the light receiver, light emitted from the light emitter cannot be received by the light receiver. The remainder of ink in the ink container can be detected on the basis of which light receiver receives light. In this system, the remainder of ink can be detected in a plurality of stages, e.g., the ink container is full, the remainder of ink is not smaller than a predetermined amount, or the remainder of ink is smaller than the predetermined amount.
The ink container is generally provided with an elastic ink scraper portion mounted along the rim of the piston to better scrape the ink off the inner surface of the cylinder. However, when a gap is produced between the piston and the inner surface of the cylinder due to, for instance, deformation of the cylinder, a part of the ink adhering to the inner surface of the cylinder cannot be scraped off the inner surface of the cylinder and is kept on the inner surface of the cylinder. When such unsatisfactory ink scraping occurs, the residual ink on the inner surface of the cylinder deteriorates the light transmission of the cylinder, which adversely affects detection of the remainder of ink in the ink container.
Further when some ink is left on the inner surface of an empty container, the ink left on the inner surface of the empty container, which can have undergone change with time, mixes with ink newly filled into the container.
When the ink containers are recycled to reuse them to another plastic product, the ink left on the inner surface of the empty container mixes in the product.
Further, the unsatisfactory ink scraping increases the amount of wasted ink.
In view of the foregoing observations and description, the primary object of the present invention is to provide a printing ink container which can suppress generation of unsatisfactory ink scraping.
Another object of the present invention is to provide a printing ink container which can ensure a high light transmission of the cylinder, thereby facilitating detection of the remainder of ink in the ink container.
In accordance with the present invention, there is provided an ink container comprising a cylinder provided with an ink discharge port at its leading end and a piston fitted in the cylinder to be slidable along the inner surface of the side wall of the cylinder so that ink is filled into the space defined by the cylinder and the piston, wherein the improvement comprises that
resistance generated by friction between the cylinder and the piston when the piston is slid toward the ink discharge port with the ink container held empty is not lower than 1.0N.
It is preferred that the resistance be not lower than 2.5N.
Further, it is preferred that at least one annular ink scraper portion be provided on the piston to extend radially outward so that its surface facing toward the ink discharge port makes an angle not smaller than 90xc2x0 to the inner surface of the side wall of the cylinder as measured toward the ink discharge port from the surface facing toward the ink discharge port.
It is preferred that a plurality of the pistons are fitted in the cylinder.
The ink container of the present invention may be incorporated in a printing device comprising a photodetector which outputs an electric signal according to the amount of light the photodetector receives, a light projecting means which projects detecting light toward the photodetector through the side wall of the cylinder, and an ink remainder detecting means which detects the remainder of ink in the ink container on the basis of the electric signal output from the photodetector.
In this case, it is preferred that the photodetector be disposed near the trailing end of the cylinder, a plurality of the light projecting means are provided in a plurality of different positions in the longitudinal direction of the cylinder and are turned on in different manners by position, and the ink remainder detecting means detects the remainder of ink in the ink container on the basis of change in the electric signal output from the photodetector.
When the ink container of the present invention is incorporated in such a printing device, it is preferred that the resistance generated by friction between the cylinder and the piston when the piston is slid toward the ink discharge port with the ink container held empty is at least 2.5N at the portion where the light projecting means projects the detecting light.
When the resistance generated by friction between the cylinder and the piston when the piston is slid toward the ink discharge port with the ink container held empty is not lower than 1.0N, the piston can satisfactorily scrape ink off the inner surface of the side wall of the cylinder, whereby the events that the residual ink on the inner surface of the cylinder deteriorates the light transmission of the cylinder and adversely affects detection of the remainder of ink in the ink container, or the ink left on the inner surface of the empty container mixes in the product when the ink containers are recycled to reuse them to another plastic product can be avoided. Further, ink in the ink container can be fully used without running to waste.
When at least one annular ink scraper portion is provided on the piston to extend radially outward so that its surface facing toward the ink discharge port makes an angle not smaller than 90xc2x0 to the inner surface of the side wall of the cylinder as measured toward the ink discharge port from the surface facing toward the ink discharge port, ink is better scraped off the inner surface of the side wall of the cylinder, whereby generation of unsatisfactory ink scraping can be more surely avoided.
Further, when a plurality of the pistons are fitted in the cylinder, ink is further better scraped off the inner surface of the side wall of the cylinder, whereby generation of unsatisfactory ink scraping can be further more surely avoided.
We have found that the ink remainder can be accurately detected even if unsatisfactory ink scraping is generated by forming the cylinder so that the gross transmittance y [% t] to light at 900 nm of the side wall of the cylinder after ink is scraped off the inner surface of the sidewall of the cylinder satisfies formula y=ax, wherein a is a coefficient not smaller than 21 and x represents a minimum output voltage of the photodetector.
In this specification, the xe2x80x9cgrossxe2x80x9d transmittance to light at 900 nm of the side wall of the cylinder is defined to be the overall transmittance to light at 900 nm of the side wall of the cylinder and the ink left on the inner surface of the side wall, if any, and the xe2x80x9cnetxe2x80x9d transmittance to light at 900 nm of the side wall of the cylinder is defined to be the transmittance to light at 900 nm of the side wall of the cylinder free from any stain.
The coefficient a is empirically obtained on the basis of the relation between the output voltage of the photodetector and the gross transmittance of the side wall of the cylinder. For example, light is received by a photodetector through side walls of the cylinder having different transmittances and the output voltages of the photodetector are detected and plotted against transmittances of the side wall of the cylinder. Then the inclination of a straight line representing the plot is taken as the coefficient a.
Though the minimum output voltage of the photodetector varies by the performance of the photodetector, the term xe2x80x9cminimum output voltage of the photodetectorxe2x80x9d should be interpreted to be the minimum voltage that a detecting means for detecting the output voltage of the photodetector can detect.
It is preferred that the coefficient a be not smaller than 36.